Bycatch mortality can cause extirpation in four freshwater turtle species

A population viability analysis and evaluations of bycatch mitigation techniques were used to investigate the impact of bycatch mortality on the populations of four freshwater turtles (common snapper, Chelydra serpentina, eastern musk, Sternotherus odoratus, northern map, Graptemys geographica, and painted, Chrysemys picta) incidentally captured in small-scale commercial fisheries operating in Lake Opinicon, Ontario, Canada.  Two approaches to bycatch reduction were investigated. The first was the use of a bycatch reduction device (BRD) and the second was the use of effort reductions (BRSs). The BRD was an exclusion device, which was vertically oriented within the funnel of the nets, which acted as a phycial barrier preventing turtles from entering. The BRSs consisted of reducing the season length.  The population viability analysis results indicated that even low levels of additional bycatch mortality of females (annual) would either result in a reduction of the population sizes or extirpation of the local population within 500 years.  The results also indicated that the use of BRD's and/or BRSs could help reduce the risk of extirpation.  The BRD's reduced the bycatch of snapping turtles by 95-100% and 0-97% for the other three species.  BRSs can also be used to reduce turtle interactions for some species. Specifically, significantly more painted turtles were caught before May 20th, than between May 20th and June 20th. Significantly less snapping turtles were caught before May 20th. No significant differences were found for the other two species. The combined use of the BRD and BRSs would prove beneficial in this fishery.